Many internal combustion engines, such as engines operating on the four-stroke principle, have intake and exhaust valves provided in the cylinder head of the engine. The intake valves open and close to selectively communicate the air intake passages of the engine with the combustion chambers of the engine. The exhaust valves open and close to selectively communicate the exhaust passages of the engine with the combustion chambers of the engine.
To open the valves, many engines are provided with one or more camshafts having one or more cams provided thereon. The rotation of the camshaft(s) causes the cam(s) to move the valves to an opened position. Metallic coil springs are usually provided to bias the valves toward a closed position.
Although metallic coil springs effectively bias the valves toward their closed positions for most engine operating conditions, at high engine speeds, the metallic coil springs have a tendency to resonate. When resonating, the metallic coil springs cause the valves to vacillate between their opened and closed positions, which, as would be understood, causes the intake and exhaust passages inside which the valves are connected to be opened when they should be closed. This results in a reduction of operating efficiency of the engine at high engine speeds.
One solution to this problem consists in replacing the metallic coil springs with air springs. An air spring typically consists of a cylinder having a piston therein. An air chamber is defined between the cylinder and the piston. The valve (intake or exhaust) is connected to the piston of the air spring using a cotter. A spacer, in the form of a shim, is disposed between the valve and the cam to ensure that the valve seats properly in the closed position and opens properly in the open position. When the cam moves the valve to its open position, the piston of the air spring moves with the valve, thus reducing the volume of the air chamber and as a result increasing the air pressure therein. When the cam no longer pushes down on the valve, the air pressure inside the air chamber causes the piston of the air spring to return to its initial position and to return the valve to its closed position.
Air springs do not resonate at high engine speeds the way metallic coil springs do. Also, for equivalent spring forces, air springs are lighter than metallic coil springs. Furthermore, air springs have progressive spring rates, which means that the spring force of an air spring varies non-linearly depending on the position of the piston inside the cylinder of the air spring, which may also be advantageous for certain engines.
Although air springs offer many advantages over metallic coil springs, they also have some deficiencies that need to be addressed.
One of these deficiencies is that air can leak out of the air springs when the engine is not in use. When the air pressure inside the air springs becomes too low, this causes the valves to move to their opened positions. When this occurs and the engine is started, the pistons of the engine can come into contact with the valves. Even if the engine is not in use, the piston may still come into contact with the valves, for example if the crankshaft is caused to rotate as a result of towing or otherwise transporting the vehicle. The impact of the piston on the valve can potentially damage the valve, for example by dislodging the shim or the cotter from their intended positions. As a result, the operation of the valve is impaired, possibly preventing operation of the engine.
One possible solution consists in providing metallic coil springs having a relatively low spring constant in addition to the air springs. The metallic coil springs are strong enough to bias the valves towards their closed position even when the air pressure inside the air springs is no longer sufficient to do so on its own. However, these metallic coil springs do not provide enough biasing force to return the valves to their closed position fast enough while the engine is in operation. Although the addition of these metallic coil springs will prevent the pistons of the engine from coming into contact with the valves when the engine is started, they add weight and complexity to the air spring system. The additional metallic coil springs can also lead to some resonance as the speed of the engine increases.
Therefore, there is a need for an air spring assembly having a construction that maintains a secure connection between the various components thereof.